1. Field of Invention
This invention relates to a novel method of isotope purification and/or enrichment.
2. Brief Description of the Prior Art
The radiochemical industry requires methods of purifying radioactive isotopes (also stable isotopes with unusual mass) from the more abundant chemically identical isotopic forms. Most importantly, a large industry is involved in the enrichment of the isotope uranium-235 from natural uranium, 99.29 percent of which is uranium 238. To be used for powering nuclear reactors, the uranium-235 content is typically increased from 0.71 percent to 3 percent. Similarly, large scale enrichment plants exist for the isolation of the stable heavy isotope of hydrogen, deuterium, which is used in heavy water-cooled nuclear reactors.
Known processes for enrichment of uranium in the fissionable isotope uranium-235 include gaseous diffusion processes and gas centrifuge processes. Typically, such processes are multi-stage and involve significant capital outlay for equipment. For both the diffusion and the centrifuge processes the feed material is uranium hexafluoride, the only uranium-bearing compound that is gaseous at ordinary temperatures. In gaseous diffusion, the gas is blown through a series of thin, porous barriers. Because the .sup.235 U atoms are lighter and thus faster than the .sup.238 U atoms, the molecules having the .sup.235 U atoms flow through each barrier more rapidly than those comprised of .sup.238 U atoms. As such, successively richer concentrations of .sup.235 U are segregated. Gaseous diffusion suffers from the disadvantage that large energy expenditures are required to achieve enrichment. Furthermore, because the enrichment achieved in any one step is small, a large multiunit plant is required. Additionally, the low enrichment levels make it impractical to recover more than a fraction of the uranium-235.
Centrifugation also utilizes the difference in the respective masses of the isotopes. Radial separation is induced by the centrifugal force established in a vertical spinning rotor, the .sup.235 U atoms being concentrated at the core. The axial movement or separation of the two isotopes necessary for segregation is induced by a variety of precise temperature gradients and/or the presence of mechanical elements which affect the fluid dynamics of the gas in a manner which optimizes the axial separation. The net result is that the light isotope (.sup.235 UF.sub.6) accumulates at one end of the machine and the gas at the opposite end is depleted in that isotope. The basic objective in designing a centrifuge is to produce the fastest and longest rotor possible, since separation increases with speed and is proportional to length. However, the level of materials technology and engineering, in general, impose practical limits on the size and speed of the rotors. For example, the peripheral speed of the rotor is limited by the ratio of strength to density of the material it is made of. The limit of the length of the machine is determined in part by the difficulty of controlling the straightness of the rotor, the uniformity of the wall in mass production manufacture, and by the durability of the bottom bearing, which must support the weight of the rotor. Another disadvantage of the centrifuge is that those with large length to diameter ratios have a problem getting up to operational speed, since they may pass through critical speeds at which large scale vibrations can occur. These problems are compounded by the fact that typically, three or more centrifuges are utilized in series as a "cascade."
Thus, although the gas centrifuge requires substantially less energy than gaseous diffusion, establishment of plants requires a large capital outlay and their maintenance involves high costs.
Another more recently developed method of isotope separation is laser separation which utilizes the differences in the electronic structure, i.e. in the electron clouds that surround the nuclei, of isotopes. The differences, though small, affect the wavelengths of light absorbed by the isotopes, i.e. each isotope absorbs light of a slightly different color. Because a laser emits light of a very pure color, it can thus be utilized to "tag," i.e. excite the electrons of, one isotope but not the other. Based on this principle several mechanisms for laser separation of isotopes have been proposed whereby a mixure of atoms or molecules containing more than one isotope is irradiated by a laser whose wavelength has been adjusted so that it excites the atoms of one isotope but has no effect on the others. Once the various isotopes are thus distinguished a variety of methods are then available for actually sorting them into different "bins." For example, application of an electric or magnetic field may be used to deflect the excited isotope along a different path from that followed by the unexcited one. Alternatively, instead of deflecting atoms which are excited but electrically neutral, another approach to laser separation techniques converts the excited isoptopes into ions which are more easily manipulated.
Still other classes of laser separation decompose molecules containing one isotope into stable products that can be simply removed from the mixture or utilize chemical scavengers which will react only with a molecule in its excited state. In addition to the similar disadvantage of the other laser techniques, i.e. requiring specialized and costly equipment, these techniques have the added difficulty of finding compounds which decompose into stable components or of finding appropriate scavenger molecules. The laser separation techniques have considerable potential but these have yet not been fully developed. These laser techniques all suffer from the disadvantage of retrieving only a small proportion of the available isotope. Thus, these techniques will require multiple stages with the accompanying reduction in efficiency and furthermore are unlikely to achieve complete recovery of the rarer isotope.
Accordingly, there is a need for a technique for isotope separation which has a high capacity and high degree of enrichment per stage, which recovers the large majority of the desired isotope, and which does not require large sophisticated equipment.